A separation method using a filtering membrane has a lot of advantages over a method for separation through heating or phase-change. One of the advantages is the high reliability of the process which can be achieved by controlling the size of the fine pores of the filtering membrane so as to stably obtain the water of desired quality. Furthermore, since the filtering membrane does not require a heating process, it can be advantageously used for water treatment using microorganisms that could be adversely affected by heat.
As the filtration process is performed by the filtering membrane, however, the impurities are likely to cause the contamination of the filtering membrane thereby reducing the permeability thereof.
Although the cleaning of the filtering membrane such as backwashing, aeration and the like can be carried out to solve the problem of the filtering membrane contamination, such cleaning has a limitation in that it removes the impurities from the filtering membrane which has been already contaminated and thus functions only as an action after accident.
Thus, to radically minimize the contamination of the filtering membrane, it is conceivable to perform a pretreatment as an advance action to remove relatively large solid materials from the feed water before the filtration by the filtering membrane.
The pretreatment is problematic, however, because it requires a separate and additional space and facility therefore and thus increases the filtration cost.